Phase transitions are a generic, but not universal, feature of gauge field theories, like the Standard Model, which are based on elementary particle
mass generation by spontaneous symmetry-breaking [1]}, [2]}.
When there is a phase transition in a gauge theory it is (except for special parameter choices) first-order,
which means that just below the critical temperature,
the universe transitions from a metastable quasi-equilibrium state into a stable equilibrium state,
through a process of bubble nucleation, growth, and merger [3]}, [4]}, [5]}, [6]}.
Such a first-order phase transition in the early universe naturally leads to the production of gravitational waves [7]}, [8]}. If it took place
around the electroweak scale, by which we mean temperatures in the range 100 – 1000 GeV,
the gravitational wave signal could lie in the frequency range of
the upcoming space-based gravitational wave detector
LISA (Laser Interferometer Space Antenna) [9]}.
The approval of the mission, and the detection of gravitational waves [10]},
has generated enormous interest in phase transitions in the early universe.
